An OS rework would be needed so locations 0/1 don't get stomped on. These are just work areas mainly used in cold/warmstart. I think there is one XL OS (the one the spontaneously resets when you change a cart rather than doing a lockup like later ones) that can have issues if you change contents of one of those locations.
Likely the old way of halting the CPU with external circuitry would be needed like early 400/800.
The 6510 has the BA pin that I think can be used like /Halt but I think it only works once the current instruciton has finished (or similar).
Generally the C64 uses /RDY when doing DMA bursts and it needs a few cycles pre-emption since write instructions can delay it's effect.
Possibly the biggest problem - can the 6510 operate at the higher speed? I suppose if not then the CPU from a C128 could be used as it has the "Fast" mode where it operates at 2 MHz.
But overall - why? Without extensive further rework to use the 6 IO port bits as either bankswitching or something else (yep, the C64 only has 6 bits of IO, not 8 on the CPU), the CPU is functionally the same from a programming POV.
I guess one project might be to do an XL with 4 joystick ports that work with all games including those that read the ports directly. But then you'd get the OS issues and software aplenty that stomps locations 0/1.
Edited by Rybags, Wed Jul 11, 2018 2:01 AM.